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Chapter 1Introduction

Travel Epidemiology

TRAVEL EPIDEMIOLOGY

Allison Taylor Walker, Regina C. LaRocque, Mark J. Sotir

Travelers are an epidemiologically important population because of their mobility, their potential for exposure to diseases outside their home country, and the possibility that they may carry nonendemic diseases between countries. International tourist arrivals were 1.2 billion in 2015 and are projected to increase to almost 2 billion by 2030, so the public health impact of travel will likely only increase. The destinations of travelers are also changing. Increased travel to destinations in Asia (arrivals up 5% from 2014 to 2015) and the Middle East (arrivals up 3% from 2014 to 2015) and anticipated increases in travel to Africa will place more travelers at risk for a variety of travel-related conditions, including malaria, dengue, measles, and other tropical or vaccine-preventable infections.

The risk of travel-related illness varies depending on destination and traveler characteristics. Existing information regarding the actual risk for travelers (often expressed as number of events per 100,000 travelers) is limited for several reasons. It is difficult to obtain an accurate numerator (number of cases of disease among travelers) and denominator (number of travelers overall or travelers to a specific destination). Many travelers who become infected will have returned to their home countries by the time they develop symptoms so will not be included in the visited country’s surveillance data. Similarly, diseases with short incubation periods or brief durations may have resolved by the time a traveler returns home and thus may not be counted in surveillance data of the traveler’s country of origin. If the illness is mild, the traveler may never seek health care, or diagnostic tests may not be performed to accurately diagnose the cause. Travelers often visit multiple locations, and it may be difficult to determine the location in which the exposure occurred.

Frequently quoted studies on the incidence of infection in travelers are based on extrapolations of limited data collected in limited samples of travelers. Often, these studies were conducted >20 years ago and might be of limited relevance to current travelers. Furthermore, these studies use a variety of methodologic designs, each with its own set of strengths and weaknesses, making the findings difficult to compare or combine. They have also, for the most part, only examined a few key diseases or conditions and have combined all travelers regardless of destination or purpose of travel. Many have been single-clinic or single-destination studies that lead to conclusions that do not apply to groups of travelers with different local, national, or cultural backgrounds.

A number of factors are relevant to epidemiologic data on travel-related diseases and adverse health events. First, the characteristics of the disease itself must be considered, including mode of transmission, incubation period, signs and symptoms, duration of illness, and diagnostic testing. Second, the presence, frequency, seasonality, and geographic distribution of the disease need to be assessed; these might change over time because of outbreaks, emergence or reemergence in new areas or populations, successful public health interventions, or other factors. Third, travelers represent a unique subset of people, and their exposures, behaviors, and disease susceptibility might differ dramatically from those of the local population at a tourist destination.

Along with demographic characteristics, additional travel-specific factors that should be considered include trip length, destinations (both current and previous), specific travel itineraries, use of preventive measures, and purpose of travel. Fourth, travelers themselves are a heterogeneous group, and different subgroups of travelers might have different risks because of activities, behaviors, and other factors during travel. For example, travelers who are visiting friends and relatives (VFR travelers) have consistently demonstrated higher proportions of serious febrile illness, particularly malaria, when compared with other types of travelers.

During the past 2 decades, the most relevant data on travel-related disease occurrence have come from surveillance of travelers themselves. Data on disease incidence in local populations may identify the most important diseases to monitor within a country, but relevance of such data to travelers—who have different risk behaviors, eating habits, accommodations, knowledge of preventive measures, and activities—is usually limited. Surveillance data that are either focused on travelers or on illnesses that affect travelers are more useful in describing travel-related disease patterns and risks.

Data collected by the Global TravEpiNet (GTEN) network of pretravel visits in health clinics across the United States provide a snapshot into the types of travelers seeking pretravel health care and their travel practices. Health care providers need to understand the epidemiologic features of the traveling population to guide their pretravel recommendations and post-travel evaluations. In examining GTEN data collected from 2009 through 2011, 13,235 travelers ranged in age from 1 month to 94 years (median 35 years). The median duration of travel that prompted the pretravel evaluation was 14 days, although 22% of travelers were taking trips of >28 days, and 3% of the travelers were taking trips of >6 months. A total of 75% were traveling to malaria-endemic countries, and 38% were visiting yellow fever–endemic countries. Immunocompromising conditions, such as HIV infection and AIDS, organ transplantation, or receipt of immunocompromising medications, were present in 3% of GTEN travelers.

Post-travel illness surveillance data are collected by the GeoSentinel Global Surveillance Network, a worldwide data collection and communication network composed of International Society of Travel Medicine (ISTM) member travel and tropical medicine clinics. Analyses of these data are used to describe the relationships between travel and travel-related illness in specific subpopulations of travelers. A 2013 summary of GeoSentinel data found that diarrheal, febrile/systemic, and respiratory illnesses are the most common diagnoses reported. Another study found that >20% of travelers visiting malarious areas reported inconsistent or no use of malaria prophylaxis. Post-travel data from the GeoSentinel network of 53 clinics around the world collected from 2007 through 2011 indicate that Asia (33%) and sub-Saharan Africa (27%) were the most common regions where travel-related illnesses were acquired. Malaria, dengue, enteric fever, spotted-fever group rickettsioses, chikungunya, and nonspecific viral syndromes were the most frequent contributors to the acute systemic febrile illness category. Falciparum malaria was most commonly acquired in West Africa, while enteric fever was most often contracted on the Indian subcontinent; leptospirosis, scrub typhus, and murine typhus were principally acquired in Southeast Asia. Common skin and soft tissue infections, mosquito bites (often infected), and allergic dermatitis were the most common skin conditions affecting travelers. Among the more exotic diagnoses, the most important were hookworm-related cutaneous larva migrans, leishmaniasis, myiasis, and tungiasis. The relative frequency of many diseases varied by travel destination and reason for travel, and VFR travelers had a disproportionately high prevalence of serious febrile illness (malaria) and low rates of seeking advice before travel (18%). Only 40% of all ill GeoSentinel travelers reported pretravel medical visits.

Investigations of travel-related outbreaks can also provide data on epidemiologic patterns of travel-related illness. Outbreaks in travelers to the Caribbean are a reminder of travel-associated risk of illness. Incidence of chikungunya was evaluated among 102 participants returning from the Dominican Republic in 2014. Forty-two (41%) had evidence of recent chikungunya infection, and of those, 37 (90%) reported rash or joint pain. Outbreaks in travelers are sentinels to warn the international community of prevalent or unrecognized illnesses in destination countries. An outbreak investigation of sarcocystosis in travelers returning from Tioman Island, Malaysia, in 2011–2012 led to continued monitoring for additional cases and identified asymptomatic infection and late presentation. Diseases identified through medical tourism, a booming yet unregulated industry, can also highlight local transmission of health care–associated infections. Mycobacterium abscessus infections in postsurgical wounds of medical tourists returning from the Dominican Republic and Venezuela have been reported.

Familiarity with the epidemiology and prevalence of these and other infections, coupled with demographic information on travelers and their particular travel details, can help clinicians provide optimal health-related information and advice to their travelers. Clinical networks and surveillance systems provide epidemiologic data on new and prevalent global infectious disease threats. These data contribute to the evidence base in this growing field and allow for informed preparation before travel, as well as clinical awareness of travel epidemiology.